The present invention relates to an optical near-field probe of which the aperture diameter is variable by irradiation with light of an intensity to exceed a critical level.
Optical near-field probes with an aperture are widely employed in optical near-field microscopes or in the technology of optical near-field recording. The optical near-field probe mentioned above in general has a structure consisting of an optical fiber or glass capillary having a sharpened pin-point end and covered with a metallic sheath on the side surface for light shielding, of which the axial center line serves to provide a channel for light transmission and an aperture opening is provided on one end for introduction of incident light. When a laser beam is introduced into the probe at the end opposite to the pin-point sharpened end, an optical near-field spot is formed at the aperture. Since the diameter of the thus formed light spot is approximately equal to that of the aperture, an optical near-field microscope having a resolving power of the wavelength of the light or smaller can be obtained by using an aperture having a diameter not exceeding the wavelength of the light. It is also possible to conduct writing-in or reading-out of a mark having a dimension smaller than the wavelength of the light by utilizing this principle. Thus, optical near-field microscopes and optical near-field recording technology are now under way of development by utilizing the optical near-field performance.
A problem in the optical near-field probe having an aperture developed heretofore is that the aperture has a fixed dimension not to be in compliance with a variety of applications in which the diameter of the light spot is desired to be variable. For example, while a probe having an aperture of a large diameter can give a large transmissivity of light and is suitable for scanning at a high velocity, the resolving power cannot be high enough and a probe having a small aperture cannot give a high transmissivity of light not to enable scanning at a high velocity though advantageous in respect of the resolving power.
When an optical near-field microscope is used for observation of a very fine object, view field location must first be conducted with high-velocity scanning over a relatively wide area followed by high-resolution observation of a specific narrow target view field. It is of course that these processes can hardly be complied by using a single probe of a fixed dimension. The idea of using two replaceable probes having different aperture dimensions is practically not feasible because, even by setting aside the problem of troublesomeness for exchanging the probes from one to the other depending on the object of observation taking a time, it is an extremely difficult matter to ensure good centering of the view fields by exchange of the probes because of the reproducibility limit in the exactness of probe mounting.
While, as is mentioned above, the transmissivity of light through an aperture is decreased as the aperture diameter is decreased, the inventors have come to a finding that the transmissivity of light through an aperture of an extremely small diameter can be kept high when the light channel reaching the aperture has such a configuration as to ensure little attenuation of the light.
It is, however, a difficult matter to prepare a glass capillary-based probe having such a controlled configuration which must be provided with a pin-point sharpened end by drawing a glass capillary under heating. Although it is possible in principle to accomplish the configuration of the light channel having a profile as desired by the method for sharpening an end of an optical fiber-based probe by utilizing the method of anisotropic etching in which the etching process is conducted in several steps under varied conditions, this method is not suitable for obtaining probes as desired with good reproducibility.
The present invention accordingly has an object, in view of the above described problems in the prior art, to provide an optical near-field probe having an aperture of a small dimension to give a high transmissivity of light which is capable of exhibiting performance of probes having different aperture diameters.
Thus, the present invention provides an optical near-field probe having an objective end opening to which a thin film of an opaque material capable of being reversibly rendered transparent by irradiation with light is attached so as to work as a variable-aperture optical near-field probe.
In particular, the material of the thin film susceptible to the change of light transmissivity by irradiation with light is antimony or a photochromic compound.